Bipolar disorder (BD) is the Axis I condition most strongly associated with cannabis use disorder (CUD); there is a six-fold increase in the prevalence of CUD in individuals with BD relative to the general population. Individuals with co-occurring CUD and BD (CUD+BD) have substantially worse clinical outcomes than those with either BD or CUD alone. Response to traditional mood stabilizing medications appears to be poor, yet little is known about optimal treatment for CUD+BD as there have been no randomized medication trials for CUD+BD to date. Convergent evidence supports dysregulated brain ?-Aminobutyric acid (GABA)/glutamate homeostasis as a candidate target for pharmacological intervention in CUD+BD. Preclinical and clinical studies have demonstrated that CUD and BD are each associated with prefrontal GABA and glutamate disturbances and that impulsivity, a core neurobehavioral feature of both CUD and BD and a key Research Domain Criteria (RDoC) construct, is causally related to GABAergic/glutamatergic functioning. Gabapentin has been consistently shown in preclinical research to modulate GABA and glutamate transmission. In human Proton Magnetic Resonance Spectroscopy (1H-MRS) studies, both acute and chronic gabapentin dosing have been shown to increase brain GABA levels, however, few studies have investigated gabapentin effects on glutamate levels. We propose that gabapentin may impact clinical outcomes in CUD+BD individuals both directly and indirectly through their impact on impulsivity. The proposed 2-week, double-blind, crossover, proof of concept study will focus on GABA, while exploring glutamate, disturbances in CUD+BD and will evaluate: a) whether gabapentin, a medication that has been demonstrated to increase cortical GABA concentrations in healthy controls and individuals with epilepsy, may similarly act to increase dorsal anterior cingulate and basal ganglia GABA levels in individuals with CUD+BD, and b) whether increased dorsal anterior cingulate and basal ganglia GABA levels will be associated with increased functional brain activity to response inhibition (?go no-go?) cues (a well-studied neurobehavioral probe of impulsivity) as well as decreased functional brain activity to cannabis cues. Effects of gabapentin on cannabis use, mood symptoms (including anxiety and sleep), and impulsivity will be explored. Positive results may support investigation of gabapentin for the treatment of CUD+BD in large-scale, randomized clinical trials. The proposed study may also provide successful demonstration of a neurobehavioral, multimodal neuroimaging platform for evaluating the potential promise of other GABAergic drugs for CUD and/or BD, as well as other conditions marked by GABA/glutamate dysfunction.